1. Field of the Invention
The present invention relates to a polishing machine for a surface of a flat workpiece such as a semiconductor wafer of silicon single crystal, and a method of dissipating heat from such a polishing machine.
2. Description of the Prior Art
Recent years have seen semiconductor devices that are fabicated into high-density integrated circuits by the ever-advancing technology of defining intricate patterns in microscopic scale on simiconductor wafer surfaces. Designs on semiconductor devices that are available today have a line width ranging from 1 .mu.m to 0.5 .mu.m or even smaller.
Unless semiconductor wafers which serve as substrates of such semiconductor devices have flat surfaces, they cannot be processed highly accurately by various semiconductor microcircuit fabrication processes including lithography, etching, and thin-film deposition. Naturally, as the interconnections to be formed on semiconductor wafers are required to be narrower, the semiconductor wafers should have flatter surfaces. Therefore, polishing processes and polishing machines for polishing semiconductor wafers to a flat finish are also required to be improved at all times.
FIG. 24 of the accompanying drawings schematically shows a conventional polishing machine for polishing a semiconductor wafer.
As shown in FIG. 24 the polishing machine has a disc-shaped reference table 55 with a flat upper surface which is supported on a reference table holder 56. The reference table holder 56 has an integral shaft 57 coupled to a rotary actuator (not shown) for rotating the reference table holder 56. The flat upper surface of the reference table 55 is substantially fully covered with an abrasive cloth 58. A wafer holder head 8 with a semicoductor wafer 7 held against its lower surface can be rotated about its own axis by another rotary actuator (not shown). The polishing machine also has an abrasive compound supply unit 59 for supplying an abrasive compound 9 to a position between the semiconductor wafer 7 and the abrasive cloth 58. The abrasive compound 9 may comprise, for example, a fluid dispersion that is composed of an abrasive grain such as colloidal silica distributed in an alkaline solution.
The reference table holder 56 has an upwardly opening coolant reservoir 60 defined in its upper surface and closed by the lower surface of the reference table 55. The shaft 57 has a coolant supply passage 61 and a coolant discharge passage 62 which are defined therein in communication with the coolant reservoir 60. The coolant supply passage 61 and the coolant discharge passage 62 are connected to a cooler 63 and a coolant supply 64. The coolant supply 64 supplies a coolant to the cooler 63 which cools the coolant. The coolant cooled by the cooler 63 is supplied through the coolant supply passage 61 into the coolant reservoir 60. After having cooled the reference table 55, the coolant is discharged from the coolant reservoir 60 through the coolant discharge passage 62 back to the coolant supply 64 so that the coolant will be used in circulation.
To polish the semiconductor wafer 7 highly flatwise, it is necessary for the reference table 55 including the abrasive cloth 58 to have a flat surface that is pressed against the semiconductor wafer 7 during the polishing process, and also to be free from abrasive wear and deformation due to mechanical stresses.
To meet the above requirements, the reference table 55 is made of a material and has a structure such that the reference table 55 has a desired mechanical strength. If the semiconductor wafer 7 has a relatively large diameter, or the polishing machine is relatively large in size or operates at relatively high speed to increase its ability to polish the semiconductor wafer 7 for higher productivity, then the reference table 55 tends to be deformed by a localized temperature irregularity thereof due to a friction-induced heat generated in a local region where the semiconductor wafer 7 is in abrasive contact with the reference table 55. Such a deformation will prevent the semiconductor wafer 7 from being polished to a desired degree of flatness. To polish the semiconductor wafer 7 highly efficiently, it is necessary that the semiconductor wafer 7 be polished at high speed while being pressed against the reference table 55 under strong forces. However, such a high-speed, high-pressure polishing process results in an increase in the temperature of the semiconductor wafer 7 and the abrasive cloth 58, increasing the localized temperature irregularity of the reference table 55.
To achieve a desired degree of flatness of the semiconductor wafer 7, the semiconductor wafer 7 and the abrasive cloth 58 be held in uniform contact with each other. More specifically, during the polishing process, the friction-induced heat is generated between the semiconductor wafer 7 and the abrasive cloth 58, heating them to a higher temperature. Unless the contacting surfaces of the semiconductor wafer 7 and the abrasive cloth 58 were kept at a uniform temperature, it would not be possible to polish the semiconductor wafer 7 to a uniform surface finish.
The polishing capability of the abrasive compound 9 also depends on the temperature thereof. If the temperature of the abrasive compound 9 present between the semiconductor wafer 7 and the abrasive cloth 58 becomes irregular, the abrasive compound 9 can no longer polish the semiconductor wafer 7 to a uniform surface finish.
The coolant reservoir 60 serves to cool the reference table 55 to prevent the semiconductor wafer 7 and the abrasive cloth 58 from being unduly heated. FIG. 25 of the accompanying drawings shows a temperature distribution across the reference table 55 and the reference table holder 56. As shown in FIG. 25, the region where the reference table 55 and the semiconductor wafer 7 are held in abrasive contact with each other has a relatively large flow of frictional-heat energy directed downwardly as indicated by the arrow A, and a relatively small flow of frictional-heat energy directed downwardly as indicated by the arrow B near the circumferential edge of the reference table 55. The same abrasive-contact region also has an upward flow of heat energy, as indicated by the arrow C, from the rotary actuator which rotates the shaft 57 of the reference table holder 56. As a result, as shown on the lefthand side of FIG. 25, the abrasive-contact region on the reference table 55 contains an area that undergoes relatively high frictional heat as indicated by the solid line and an area which undergoes relatively low frictional heat as indicated by the dotted line.
Since the reference table 55 usually has a thickness of several tens millimeters, only the coolant reservoir 60 cannot sufficiently cool the frictional face side of the reference table 55. As a consequece, the temperatures of the face and reverse sides of the reference table 55 differ widely from each other, causing the reference table 55 to be largely deformed as shown in FIG. 26 of the accompanying drawings. The reference table 55 is normally made of SUS or a ceramic material, whereas the reference table holder 56 of cast iron. The reference table 55 is therefore also deformed due to different coefficients of thermal expansion of the reference table 55 and the reference table holder 56. For the above reasons, the reference table 55 cannot keep its face side as flat as desired for uniformly polishing the semiconductor wafer 7.
FIG. 28 of the accompanying drawings shows a process of polishing one semiconductor wafer 7 at a time with the abrasive cloth 58, and FIG. 29 of the accompanying drawings shows a process of polishing a batch of four semiconductor wafers 7 supported on a single wafer plate 65 with the abrasive cloth 58. In either of the illustrated processes, the heat generated in the region where the reference table 55 is in sbrasive contact with the semiconductor wafer or wafers 7 is responsible for a temperature irregularity on the surface of the reference table 55, and the abrasive cloth 58 imposes an abrasive load on the semiconductor wafer or wafers 7 in that region due to the abrasive action of the abrasive cloth 58 on the semiconductor wafer or wafers 7 during rotation of the abrasive cloth 58. In FIGS. 28 and 29, as the curve goes higher, the abrasive load is higher and so is the frictional head. Therefore, as shown in FIG. 27 of the accompanying drawings, the center of each semiconductor wafer 7 is higher in temperature than the circumferential area thereof, resulting in an irregular temperature distribution in each semiconductor wafer 7. Irrespective of whether one semiconductor wafer is polished at a time or a batch of semicondutor wafers 7 are polished simultaneously, it has been impossible to finish the semiconductor wafer or wafers 7 to a desired flat finish.